Aliens (XCOM: Enemy Unknown)

"Aliens" (often referred to as "X-Rays" by the soldiers) is the Human term used to collectively designate the extraterrestrial enemy force that invades Earth in 2015 under the direction of the Uber Ethereal.

The aliens rely on several variants of plasma weaponry ranging from the wrist-mounted Sectoid pistols to the Elite Muton's massive plasma cannons. These variations boil down to the same four basic weapons of increasing power and size:

However, the aliens lack specialist weaponry for either long-range marksmanship or close-range assault but conceivably they have the ability to produce such weaponry as proven by XCOM's rapid development of the Plasma Sniper Rifle and Alloy Cannon designs. For additional assault firepower, elite alien troops use Alien Grenades, which are more powerful than their Human equivalent, yet have the same blast radius. All alien small arms are set to self-destruct after their user's death, although they may leave behind Weapon Fragments which can provide basic insight into the technology.

Chryssalids are living weapons themselves, ripping apart their targets to shreds with their poisonous claws or implanting their victims with their eggs.

Zombies, an intermediary between a healthy humanoid and a (newborn) Chryssalid is a surprisingly sturdy and similarly medium-heavy hitting monstrosity.

Seekers (EW) are floating cloaked isolator hunters who disrupt your squad construction, gamplay planning and use a weak yet continual unique strangulation martial concept which is most dangerous if your are not prepared to deny it while dealing with others.

To carry this out, the aliens begin space-borne intrusions on Earth rather than an all-out apocalyptic assault, initially aiming to abduct random civilians to carry out basic experiments. After the XCOM response becomes tangible, the force begins conducting overt "Terror Missions" in populated areas as well as relying on various other means of raising worldwide panic that seems to aim to undermine support to XCOM. However, the Ethereals kept leaving clues that finally brought the most able, most psionically-gifted human onto their capital ship.

With the exception of ship commanders, new enemy units begin to be deployed by the alien forces according to a fixed schedule (see table, below). Species that are introduced in the early and mid-game are generally most predominant during the first one to three months that they appear. Early-game species such as Floaters drop to a very low chance of deployment (circa 2.5%) by late-game, but are never removed from the pool of available aliens. Outsiders are an exception, as they no longer appear following the Alien Base Assault mission.

Operation Progeny and Slingshot DLC missions do not adhere to the schedule and may include aliens before they are regularly deployed (e.g. Mutons can appear in Slingshot missions before May). Similarly, Ethereals may be encountered during the XCOM Base Defense mission in Enemy Within. These occurrences are unique, and do not prematurely commence the appearance of these aliens in other missions.

Within the first seven months all of the non-commander alien species are deployed during regular gameplay. The appearance of new aliens is delayed if the "Marathon" Second Wave option is enabled, indicated by a month in brackets in the table below.

Any alien that is not Hardened should be able to use the Hunker Down ability, but cannot due to a bug in the XComGame.upkgame file. This can be fixed using a hex editor to manually change data in the file. Details can be found in The Nexus Forums, here. Hunker Down can be a random behaviour for panicked aliens (and humans).